We propose to explore the chemical and structural events in myosin which are essential to the contraction of muscle. We will study the conformational states and transitions which occur in myosin when it interacts with MgATP and actin. Our main approach will be to monitor the conformational changes by chemical modification of the two sulfhydryl groups of myosin, SH1 and SH2 with bifunctional reagents of different length. The reactivity and the separation of these groups can sensitively monitor the changes in the active site of myosin. We will also investigate the role of the SH1 and SH2 groups in the formation of the protein-substrate complex. (ATP binding, thiol modification and affinity labeling experiments will be carried out). We will study the interactions between myosin heads in the presence of MgATP and actin. In these experiments we will use myosin with the SH1 - SH2 groups bridged by pPDM. We will employ immunological methods, proteolytic digestion and nanosecond fluorescence decay measurements to explore the structural and functional role of myosin DTNB light chain in the contraction of muscle.